1. Field of the Invention
This invention relates to an aromatic polyester copolymer with improved stability and, more particularly, to an aromatic polyester copolymer with improved durability to aging under dry and moist heat (hereunder referred to as durability under dry and moist heat) as well as improved resistance to water crazing (hereunder crazing resistance).
2. Description of the Prior Art
Aromatic polyester copolymers comprising a mixture of terephthalic acid and isophthalic acid or functional derivatives thereof (the molar ratio of terephthalic acid to isophthalic acid being 9:1 to 1:9) and a bisphenol have been known for many years. Such aromatic polyesters are produced by "interfacial polymerization" wherein an aqueous alkali solution of a bisphenol is mixed with an aromatic dicarboxylic acid chloride dissolved in an organic solvent immiscible with water (W. M. Earechson, J. Poly. Sci., 40 399 (1959) and Japanese Patent Publication No. 1959/65), "solution polymerization" wherein a mixture of a bisphenol and acid chloride is heated in an organic solvent (A. Conix, Ind. Eng. Chem., 51 147 (1959) and U.S. Pat. No. 3,133,898), "melt polymerization" wherein a mixture of a phenyl ester of an aromatic dicarboxylic acid and bisphenol is heated (British Pat. No. 924,607 and U.S. Pat. No. 3,399,170), and other methods. According to one example of the interfacial polymerization which is a typical polymerization method, a solution of a mixture of terephthaloyl dichloride and isophthaloyl dichloride in methylene chloride is mixed with an aqueous solution of bisphenol A in caustic soda and polymerized under stirring at 5.degree. to 25.degree. C. for about 5 minutes to 3 hours. The resulting aromatic polyester copolymer usually contains at least about 100 mol eq./ton of the terminal carboxyl.
It is also well known that the aromatic polyester copolymer thus-prepared from aromatic dicarboxylic acids and bisphenols is superior to the polyester comprising aromatic dicarboxylic acids and aliphatic alkylene glycol with respect to mechanical properties such as tensile strength and elongation, flexural strength, bend recovery, and impact strength, physical properties such as thermal distortion temperature and dimensional stability, electrical properties and fire retardancy. Therefore, the aromatic polyester copolymer is used in various fields as articles fabricated by extrusion molding, injection molding and other molding methods, films, fibers, and coating materials. The advantages of such aromatic polyester over the polyethylene terephthalate and polybutylene terephthalate are its transparency, a high thermal distortion temperature (160.degree. to 180.degree. C.) and high fire retardancy. The aromatic polyester copolymer finds much utility where high temperature atmosphere under dry heat or moist heat is predominant.
However, one of the problems with such an aromatic polyester copolymer is that if it is exposed to high temperature atmosphere under dry or moist heat for an extended period, it discolors or its mechanical characteristics are impaired. Another problem that is observed is that if it is placed in hot water, like polycarbonate, the copolymer undergoes crazing wherein fine voids are formed inside of the copolymer which render it opaque and impair its mechanical characteristics.
Heretofore, many methods have been proposed to solve these problems. For example, a method involving mixing and melting polyethylene terephthalate with the aromatic copolyester (e.g., as described in U.S. Pat. No. 3,946,091), a method comprising mixing and melting polyethylene hydroxybenzoate with the aromatic copolyester (e.g., as described in U.S. Pat. No. 3,884,990), a method involving mixing and melting polybutylene terephthalate with the aromatic copolyester (e.g., as described in Japanese patent application (OPI) No. 34342/76 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application")), a method comprising mixing and melting an organic phosphite ester with the aromatic copolyester (e.g., as described in U.S. Pat. No. 4,097,431), a method comprising mixing and melting an organic phosphate with the aromatic copolyester (e.g., as described in U.S. Pat. No. 4,123,420) and a method comprising mixing and melting an epoxy compound with an aromatic copolyester, are known.
Although the addition of polyethylene terephthalate, polyethylene hydroxybenzoate or polybutylene terephthalate is effective for providing improved crazing resistance, it is prone to reduce the inherent high fire retardancy of the aromatic polyester copolymer, and it has little effect to improve the durability under dry and moist heat. The addition of a phosphorous compound, in many cases, decreases the inherent high fire retardancy of the aromatic polyester copolymer, lowers the transparency of the copolymer due to its poor miscibility with the phosphorous compound, and what is more, the addition is not much effective to improve the durability under dry and moist heat or crazing resistance. Some of the epoxy compounds are effective to some extent in improving the durability of the copolymer under dry and moist heat, but they are not fully effective for providing improved crazing resistance and other than that, they have a strong tendency to accelerate coloring of the aromatic polyester copolymer during melt molding, thus reducing its commercial value considerably.